1. Field of the Invention
The present invention relates in general to sensors for sensing a rotation angle, and more particularly to sensors that sense a rotation angle or open degree of a throttle valve installed in an air intake manifold of an internal combustion engine.
2. Description of the Background Art
Generally speaking, a throttle valve is installed in an air intake manifold of an internal combustion engine, and a rotation angle sensor detects an open angle of the throttle valve. A detecting signal of the rotation angle sensor is routed to an engine control unit, which signal indicates an intake air amount of the engine. The engine control unit calculates a fuel injection amount corresponding to the intake air amount. The rotation angle sensor has a potentiometer, which is comprised of a resistant element and a brush for detecting the open angle of the throttle valve. The brush is connected with a rotational shaft (a throttle shaft). When the brush moves on the resistant element, the open angle of the throttle valve can be detected by a resistant value change of the resistant element. However, because the brush may be isolated from the resistant element, the detecting signal of the rotation angle sensor has the potential for being suddenly interrupted. Also, when the brush or the resistant element is used for a long period of time, the rotation angle sensor cannot keep its durability and life because of the wearing away of the brush or the resistant element.
A Japanese Patent Application First Publication No. Heisei 9-189509, published on Jul. 22, 1997, exemplifies a previously proposed rotation angle sensor with three poles, having a first yoke, a second yoke, and a third yoke surrounding a magnet, and a first Hall element installed between the first yoke and the third yoke, and a second Hall element installed between the second yoke and the third yoke.
In the disclosed Japanese Patent Application First Publication, the rotation angle sensor is shown in FIG. 9. A throttle body 1 has a throttle valve (not shown) which rotates with a valve shaft 2. The throttle body 1 has also a fit portion 1A, which forms a cylindrical shape surrounding the projection end of the valve shaft 2. The fit portion 1A is fixed to a cylindrical portion 3A of a yoke mounting member 3. The valve shaft 2 is made of a high strength metal material, which is installed on the bearings in the throttle body 1.
The yoke mounting member 3 includes a casing in conjunction with the throttle body 1, which has the cylindrical portion 3A opening in the downward direction, a partition portion 3B covering on the top end of the cylindrical portion 3A, a circuit board installing space 3C on the upper portion of the partition portion 3B, a magnet installing space 3D on the lower portion of the partition portion 3B, and a connector portion 3E projecting in the outside diameter direction of the cylindrical portion 3A.
A magnet 4 is installed in the magnet installing space 3D of the yoke mounting member 3. The magnet has circular ended portions in the lengthwise direction, parallel ended portions in the width direction, and a through hole 4A extending in the axial direction of the valve shaft 2. By inserting and crimping the projecting end of the valve shaft 2 into the through hole 4A, the magnet 4 is fixed to the valve shaft 2.
The first yoke 5 is installed in the yoke-mounting member 3. The first yoke 5 has a circular magnet portion 5A opposed to the circular portions of the magnet 4, and an extending portion 5B extending from the magnet portion 5A to the circuit board installing space 3C, and shaped like a letter L. The extending portion 5B covers a Hall element 8 from above. The extending portion 5B of the first yoke 5 leads a magnetic flux from the magnet 4 to the first Hall element 8.
The second yoke 6 is installed in the yoke-mounting member 3. The second yoke 6 has a circular magnet portion 6A opposed to the circular portions of the magnet 4, and an extending portion 6B extending from the magnet portion 6A to the circuit board installing space 3C, and shaped like a letter L. The extending portion 6B covers a Hall element 9 from above. The extending portion 6B of the second yoke 6 leads a magnetic flux from the magnet 4 to the second Hall element 9.
The third yoke 7 is installed in the yoke-mounting member 3. The third yoke 7 has a circular magnet portion 7A opposed to the circular portions of the magnet 4. An extending portion 7B extends from the magnet portion 7A along the surface of a circuit board 10 on the partition portion 3B of the yoke mounting portion 3, which extending portion 7B is installed below the extending portions 5B, 6B.
The first Hall element 8 for a first signal output means is installed on the circuit board 10, which circuit board is positioned between the extending portion 5B of the first yoke 5 and the extending portion 7B of the third yoke 7. The first signal is output proportional to a magnetic flux density through a first closed flux path which is comprised of the magnet 4, the first yoke 5, and the third yoke 7.
The second Hall element 9 for a second signal output means is installed on the circuit board 10, which stands between the extending portion 6B of the second yoke 6 and the extending portion 7B of the third yoke 7. The second signal is output proportional to a magnetic flux density through a second closed flux path which is comprised of the magnet 4, the second yoke 6, and the third yoke 7.
The circuit board 10 is installed in the circuit board installing space 3C, which includes an insulating material such as a ceramic. The Hall elements 8,9 are positioned on the circuit board 10. The circuit board 10 is connected to the end of more than one terminal 11, which extends toward the connector 3E. The circuit board 10 is connected to a signal processing circuit (not shown) by the terminal 11.
The circuit installing space 3C of the yoke-mounting member 3 is sealed with a cover 12, which may be a resin plate. Packing 13 is installed between the cover 12 and the circuit installing space 3C, which packing includes an elastic member.
A magnetic shielding plate 14 is buried in the partition portion 3B of the yoke mounting member 3, which interrupts a leakage flux and reduces the effect of a leakage magnetic field to the Hall elements 8,9.
In the above-mentioned the rotation angle sensor, when the valve shaft 2 rotates to the throttle valve openings, the magnet 4 which is fixed on the valve shaft 2 also moves. Then, because each area ratio of the circular portion of the magnet 4 relative to the circular magnet portion 5A or the circular portion of the magnet 4 relative to the circular magnet portion 6A changes, the magnetic flux density through the yoke 5,6 changes.
Each Hall element 8,9 outputs a signal proportional to the magnetic flux density to the signal processing circuit through the circuit board 10 and terminal 11. Therefore, the valve shaft opening is detected by the signal processing circuit.
Incidentally, in the above-mentioned rotation angle sensor, because the magnetic shielding plate 14 is buried in the partition portion 3B of the yoke mounting member 3, this requires preparations for many parts. Also, when the yoke-mounting member 3 is made, because the magnetic shielding plate 14 must be molded in the partition portion 3B, it causes inconvenience in the workability.
In addition, the Hall elements 8,9 are contained on the circuit board 10 of the circuit installing space 3C, which is located above the partition portion 3B of the yoke mounting member 3. When the yoke-mounting member 3 is mounted on the cylindrical fit portion 1A of the throttle body 1, it results in an undesirably increased size of the sensor.